Communication systems that utilize coded communication signals are known in the art. One such system is a direct sequence code division multiple access (DS-CDMA) cellular communication system, such as set forth in the Telecommunications Industry Association Interim Standard 95B (TIA IS-95B) herein after referred to as IS-95. In accordance with IS-95, the coded communication signals used in the DS-CDMA system comprise spread spectrum signals which are transmitted in a common 1.25 MHz bandwidth channel between mobile communication units and base transceiver stations (BTS) located at base sites of the wireless communication system. Each 1.25 MHz bandwidth portion of the radio-frequency (RF) spectrum, or 1.25 MHz bandwidth channel, carries spread spectrum signals centered around a particular carrier frequency and is commonly referred to as a narrowband DS-CDMA channel. Information from a mobile communication unit is modulated by the carrier frequency within the 1.25 MHz bandwidth channel by means of orthogonal waveforms, and without benefit of a pilot signal. Accordingly, recovery of the spread spectrum signals by a BTS is enabled via the use of well known, non-coherent demodulation techniques.
A mobile communication signal transmitted from a mobile communication unit to a BTS, may be reflected off of nearby scatterers, such as buildings, and result in multipath propagation of the transmitted signal. These reflections produce replicas, typically referred to as multipath replicas, of the originally transmitted signal which arrive at a base site receiver with various power levels at various times. The power levels, commonly referred to as path energies are determined by propagation distances traveled by the multipath replicas as well as environmental conditions. Upon receipt by the BTS, the originally transmitted signal and its multipath replicas are filtered, correlated, despread, recombined and decoded to yield the desired voice or data signal.
For purposes of discussion, the BTS includes a base station receiver assembly which includes a multipath signal searcher and a spread spectrum receiver. The presence of multipath replicas is typically detected using the multipath signal searcher comprised of multiple searcher paths searching at multiple time offsets. The ability of the multipath signal searcher to detect the multipath replicas is determined in part, by a received path energy of the multipath signal. Each received path energy yields a path energy metric as a result of estimating Walsh symbol energies by the searcher path. Accordingly, prior art multipath signal searchers utilize only a plurality of path energy metrics to direct selection of multipath signals for non-coherent demodulation by the spread spectrum receiver.
The received path energy is proportional to the operating Eb/Io (energy per bit divided interference spectral density) or signal-to-noise ration (SNR) of the receiver. Therefore by lowering the operating Eb/Io of the spread spectrum receiver, as can be done using quasi-coherent instead of non-coherent demodulation, a desirable increased system capacity and transmission range may result. Unfortunately, the advantage of lowering Eb/Io operation of the spread spectrum receiver is compromised when the multipath signal searcher is unable to detect incoming multipath replicas. Accordingly, less total signal energy is available for demodulation and decoding, and therefore a less robust signal, representative of the transmitted mobile communication signal, is produced.
Therefore, a need exists, for a method and apparatus to provide a signal search capability within an IS-95 wireless communication system that is easy to implement and overcomes low multipath signal detection in spead spectrum receivers.